I. Field of the Invention
The present invention relates to a method and a device for aiding the piloting of an aircraft, which are intended to aid in respecting a time of arrival constraint at a waypoint during a phase of descent of the aircraft.
II. Description of Related Art
Piloting of aircraft, especially of modern civil transportation aircraft, generally takes place along a flight plan composed of a set of points of arrival (“waypoints” in English) defined in three-dimensional space. The aircraft must be guided along the flight plan by respecting a maximum margin of error of position relative to segments connecting the different waypoints. More and more often, especially in zones of high traffic density, at least some of the waypoints of the flight plan involve a time of arrival constraint, generally known as RTA (“Required Time of Arrival” in English), imposed by the air traffic controller in order to guarantee satisfactory separation of different aircraft. In such a case, the aircraft must be guided along the flight plan by additionally respecting a maximum margin of error in time relative to the RTA constraints on the different waypoints involving such a constraint. Because of the fact of convergence of different aircraft, the zones of approach to airports, in which the aircraft are often in a phase of descent, often correspond to zones of high traffic density in which the air traffic controller tends to impose RTA constraints. Consequently, the aircraft must be capable of respecting a time of arrival constraint RTA at a waypoint corresponding in a flight plan to a phase of descent.
In modern aircraft, the pilots may enter time of arrival constraints RTA associated with waypoints into a flight plan within a flight management system of FMS type (“Flight Management System” in English). The FMS formulates a speed profile associated with a reference trajectory, taking each RTA constraint into account. For this purpose it takes into account the flight conditions known during formulation of the said speed profile, especially the predictions of wind, temperature, etc. along the said trajectory.
U.S. Pat. No. 5,121,325 describes a system for determining a range of estimated times of arrival of an aircraft at a particular point. These estimated times of arrival are generally referred to as ETA (“Estimated Time of Arrival” in English). The earliest time of arrival ETAmin and the latest time of arrival ETAmax correspond to the cases in which the airplane flies the scheduled trajectory at respectively the maximum and minimum possible speeds over this trajectory. The estimated times of arrival are calculated as a function of the flight conditions, especially wind, known to the FMS.
In modern aircraft, during insertion of an RTA constraint into a flight plan, the FMS determines a speed profile for the scheduled trajectory, such that an estimated time of arrival ETA calculated for this speed profile corresponds to the RTA constraint at the waypoint under consideration. Nevertheless, it is frequent that the real flight conditions while the aircraft is flying the trajectory differ substantially from the flight conditions known to the FMS. In particular, the wind intensity and/or direction may have varied compared with the forecasts. Consequently, the calculated speed profile does not permit the RTA constraint to be respected. For example, a headwind of intensity greater than that forecast value slows the forecast progress of the aircraft: the time at which the aircraft arrives at the waypoint under consideration would be later than the RTA constraint if it continued to fly according to this speed profile. With a view to respecting the RTA constraint when the difference between the estimated time of arrival ETA corresponding to the current speed profile and the RTA constraint is greater than a predetermined threshold, the piloting systems of the aircraft calculate a new speed profile of the aircraft so that the estimated time of arrival ETA corresponds to the RTA constraint. In the aforesaid example, they increase the speed of the aircraft. However, that sometimes has the consequence of shifting the speed profile to the maximum or minimum speed possible over the trajectory: the RTA constraint may then be located outside the range of possible arrival times [ETAmin, ETAmax], meaning that it is no longer possible to respect this time of arrival constraint.